The present invention relates to an alloy used for producing a high-performance Rxe2x80x94Txe2x80x94B sintered magnet and a method for producing the sintered magnet. More particularly, the present invention relates to a raw-material alloy, which is used for producing a high coercive-force Rxe2x80x94Txe2x80x94B sintered magnet which is used, in turn, mainly for a motor or the like, and it relates to a method for producing such sintered alloy.
The Rxe2x80x94Txe2x80x94B sintered magnet, in which R is at least one rare-earth element including Y, and T is Fe or a transition element, a part of which may be replaced with one of or both Co and Ni, is a representative high-performance magnet. The Rxe2x80x94Txe2x80x94B sintered magnet is indispensable functional material, which supports miniaturizing, weight reducing and performance-enhancing of the magnet-utilizing parts. The Rxe2x80x94Txe2x80x94B sintered magnet is applied in a broad field, such as electronics manufacture, various motors for OA, FA, diagnosis apparatuses for medical use and the like. Recently, the Rxe2x80x94Txe2x80x94B sintered magnet is used in various motors for automobiles.
The Rxe2x80x94Txe2x80x94B sintered magnet consists of a ferromagnetic. R2T14B phase, on which the magnetism is based, R-rich-phase (the non-magnetic phase with high concentration of a rare-earth element such as Nd), and B-rich phase (the B-rich non-magnetic phase, for example, Nd1.1FeB4 phase in a case where R is Nd).
The raw-material alloy, which is used for producing the Rxe2x80x94Txe2x80x94B sintered alloy, also usually consists of the R2T14B phase, the R-rich phase and the B-rich phase. Among these phases, the R-rich phase plays the role of supporting the liquid-phase sintering. This phase plays, therefore, an important role of enhancing the characteristics of the sintered magnet and is, hence, indispensable. Since the R-rich phase is easily oxidizable, it is oxidized in the production steps of the sintered magnet. The R content of the sintered alloy is considerably more than that of the R2T14B, which is 11.8 at %. This enables an effective R-rich phase, even after the oxidation, to remain at a certain greater level during the sintering.
On the other hand, the volume fraction of the R2T14B phase, i.e., the ferromagnetic phase, must be increased as the performance of the sintered magnet is more enhanced. This, in turn, leads to decrease in the volume fraction of R-rich phase. Therefore, when the raw-material alloy is cast by the block mold casting method, the R-rich phase detrimentally disperses in the ingot and becomes locally insufficient. When the raw material powder crushed from such ingot is used for the sintered magnet, it is difficult to obtain satisfactory magnetic properties.
Meanwhile, the dendritic xcex1 Fe phase is more likely to form in the alloy which has a higher composition of the R2T14B phase volume fraction. This xcex1 Fe phase dedrimentally impairs the crushability of the raw-material alloy, so that the composition of the crushed powder varies. The magnetic properties of the sintered magnet are lowered and increasingly disperse as well. A considerable amount of the xcex1 Fe phase can be diminished by means of heat treating the raw material in inert gas, such as Ar, or under vacuum at 1000xc2x0 C. or higher for an extended time. However, upon application of this heat treatment, since the dispersion of the R-rich phase is impaired, the magnetic properties cannot be improved.
Such problems are involved in the production of a high-performance sintered magnet. A strip casting method is proposed to solve these problems (for example, Japanese Unexamined Patent Publication (kokai) No. 5-22488 and Japanese Unexamined Patent Publication (kokai) 5-295490). This method resides in the production of an alloy by means of feeding melt on the surface of a rotary roll, while the circumferential speed of the roll and melt-feeding amount are controlled to produce a thin strip alloy having from approximately 0.1 to 0.5 mm of average thickness. Therefore, this method enables a higher cooling speed in solidification than in the conventional block-mold casting method. It is, therefore, possible to finely disperse the R-rich phase and to suppress formation of the dendritic xcex1 Fe phase in the alloy produced. By means of this method, no dendritic xcex1 Fe phase can be formed in the alloy, for example in the case of Nd-Fe-B based alloy, as long-as Nd content is up to approximately 28.5% by weight.
Meanwhile, a two-alloy mixing method has been proposed (for example Japanese Unexamined Patent Publication (kokai) No. 4-338607), such that an Rxe2x80x94Txe2x80x94B alloy with a low R content (hereinafter referred to as xe2x80x9cthe main-phase alloyxe2x80x9d) and an Rxe2x80x94T or Rxe2x80x94Txe2x80x94B alloy with a high R content (hereinafter referred to as xe2x80x9cthe boundary-phase alloyxe2x80x9d) are prepared separately, and are mixed to produce a sintered magnet. By means of adding Co to the boundary-phase alloy, chemically stable R3(Fe.Co) is formed and suppresses oxidation of the boundary-phase alloy during the production of a sintered magnet (Japanese Unexamined Patent Publication (kokai) No. 7-283016).
When the fine powder of the Rxe2x80x94Txe2x80x94B alloy is slightly surface-oxidized, it is not suddenly oxidized even on exposure to ambient air, and can, therefore, be shaped under a magnetic field in ambient air. Fine pulverization is usually carried out in the production of a sintered magnet. A jet mill is used, for example, for the fine pulverization in the inert-gas atmosphere, in which trace amount of oxygen is incorporated. The thus produced fine powder of from 4000 to 10000 ppm of oxygen concentration can be shaped under the magnetic field in ambient air.
However, the permissible oxygen concentration to avoid lowering the magnet properties is lower in a high-performance sintered magnet with lower R content and hence, a less R-rich phase. It is, therefore, impossible to oxidize the surface of fine powder as described above, since the less R-rich phase must be effectively utilized. The shaping under the magnetic field must be carried out, while taking such measures as mounting the entire metal die in a glove box, establishing the protective gas atmosphere of N2 and Ar in the glove box, and carrying out the magnetic-field shaping in the glove box. In the other steps, the causes of the oxidation must be eliminated as much as possible. The cost is accordingly increased.
Meanwhile, it is necessary to suppress the grain size to approximately 10 to 30 xcexcm so as not to decrease the coercive force and squareness ratio of the sintered magnet. However, when the oxygen concentration of the sintered magnet is suppressed to an extremely low level, abnormal growth of crystal grains is likely to occur during the sintering, occasionally up to approximately 1 mm.
The present inventors considered a raw-material alloy, which is difficult to be oxidized and to undergo the abnormal growth of crystal grains in the production process of the sintered magnet, and which is used for producing a high-performance Rxe2x80x94Fexe2x80x94B sintered magnet. They also considered a method for producing said sintered magnet. More particularly, the present inventors considered a raw-material alloy, which is used for producing a high coercive-force rare earth sintered magnet which is used, in turn, mainly for a motor or the like. They also considered a producing method of such sintered magnet. As a result, the discovery was made that, when the sintered magnet is produced by a two-alloy mixing method, specifically when the main-phase alloy with an R component less than that of R2T14B and the boundary-phase alloy are mixed, only slight oxidation occurs during the production process of the sintered magnet, and no abnormal growth of crystal grains occurs during the sintering. Based on this discovery, the present invention was attained.
Namely, the present invention provides a raw-material alloy used for producing an Rxe2x80x94Txe2x80x94B sintered magnet consisting of R2T14B, in which R is at least one rare-earth element including Y, and T is Fe, a part of which may be replaced with one of or both Co and Ni, and B is B (boron), a part of which may be replaced with one of or both C and N, characterized in that said R is from 10 to 11.8 at % in total of the rare earth elements consisting of from 1 to 6 at % of Dy, the balance being at least one of Nd and Pr, the B content is from 5.88 to 8.00 at %, the dendritic xcex1 Fe phase may be dispersed in the first region of the matrix, the lamellar xcex1 Fe phase is dispersed in the second region other than the first region, the total of the xcex1 Fe phase and the first region is from 0 to 10% by volume (namely, no xcex1 Fe phase may be formed, and hence this total may be 0% by volume), and the total of the second region and the lamellar xcex1 Fe phase is 5% by volume or more.
Namely, the present invention relates to a method for producing the sintered magnet by means of mixing the Rxe2x80x94Txe2x80x94B main-phase alloy, which has so little R content that essentially no R-rich phase is present and cannot be liquid-phase sintered alone, and the Rxe2x80x94T or Rxe2x80x94Txe2x80x94B boundary-alloy, which has sufficient R content to supply the R-rich phase into the present main-phase alloy. The present invention is characterized by the following (1) through (3).
(1) Main-phase alloy
Regarding the structure, the dendritic xcex1 Fe phase is dispersed and formed (described more in detail hereinbelow) in a region of the matrix consisting of R2T14 B, in which R is at least one rare-earth element including Y, and T is Fe, a part of which may be. replaced with one of or both Co and Ni, and B is B (boron), a part of which may be replaced with one of or both C and N. Such region is 10% by volume or less.
Regarding the composition, R essentially consists of Nd, Pr and Dy; their total content is from 10 to 11.8 at %, and Dy is from 1 to 6 at %. The B content is from 5.88 to 8.00 at %. The balance consists of T.
(2) Boundary-phase Alloy
This is Rxe2x80x94T alloy or Rxe2x80x94Txe2x80x94B alloy which contains 15 at % or more of R. Preferably, the Co content is 1 at % or more.
(3) Production method of sintered magnet
The main-phase alloy in an amount of 60% by weight or more and the boundary-phase alloy in an amount of 40% by weight or less are mixed to produce the sintered magnet.
The present invention is described in detail hereinafter.
The main-phase alloy according to the present invention is characterized in that it is produced by the strip casting method and is free of the easily oxidizable, lamellar R-rich phase which is present in the usually used raw-material alloy for producing the sintered magnet. Instead, the lamellar xcex1 Fe phase is formed. The oxidation during the production of the sintered magnet can, therefore, be suppressed.
The main constituent phases of the main-phase alloy of the present invention are the lamellar xcex1 Fe phase, and in addition, the R2T14 B phase and the B-rich phase. These are the other matrix phases. In addition, the dendritic xcex1 Fe phase and the dendritic R2T17 phase may occasionally be formed. In the case of formation of these phases, the composition is unbalanced so that the R-rich phase is numerously formed in the neighborhood of these phases. The present invention is described more in detail with reference to the drawings.